Numerical assessment of optoelectrical properties of ZnSe–CdSe solar cell-based with ZnO antireflection coating layer

In this work, a numerical assessment of the optoelectrical properties of the ZnO–ZnSe–CdSe heterojunction for a thin and cost-effective solar cell was made by using the PC1D simulation software. The photovoltaic (PV) properties have been optimized by varying thicknesses of the absorber layer of the p-CdSe layer, the window layer of n-ZnSe, and the antireflection coating (ARC) layer of ZnO, a transparent conductive oxide with enhanced light trapping, and wide bandgap engineering. There is a positive conduction band offset (CBO) of ΔEc = 0.25 eV and a negative valence band offset (VBO) of ΔEv = 1.2 − 2.16 =  − 0.96 eV. The positive CBO prevents the flow of electrons from the CdSe to the ZnSe layer. Further, the impact of doping concentration on the performance of solar cells has been analyzed. The simulation results reveal the increase in the efficiency of solar cells by adding an ARC. The rapid and sharp increase in the efficiency with the thickness of the window layer beyond 80 nm is interesting, unusual, and unconventional due to the combined effect of morphology and electronics on a macro-to-micro scale. The thin-film solar cell with the structure of ZnO/ZnSe/CdSe exhibited a high efficiency of 11.98% with short-circuit current (Isc) = 1.72 A, open-circuit voltage (Voc) = 0.81 V and fill factor (FF) = 90.8% at an optimized thickness of 2 μm absorber layer, 50 nm window layer, and 78 nm ARC layer. The EQE of solar cells has been observed at about 90% at a particular wavelength at 470 nm (visible light range). Around 12% of efficiency from such a thin-layered solar cell is highly applicable.


Device architecture and simulation tool
The device architecture of the purposed solar cell is as in Fig. 1. In this schematic structure, CdSe has been chosen as an absorber layer and acts as p-type material. Similarly, ZnSe has been chosen as a window layer, which acts as n-type material lying between the ZnO ARC layer and p-type material with a device area of 100 cm 2 respectively. The electrode materials of most of the semiconductors are good due to their low VBO i.e. low valence barrier and electron reflecting ability, and higher CBO. The appropriate choice of material for back contact will improve the short circuit current limit of the CdSe layer 33 . However, we have used Silver (Ag) as the back contact electrode and Aluminium (Al) as the front contact electrode as we have incorporated it in our previous work 34 .
There are various simulation software for solar cells, among them, PC1D simulation has been chosen because of free availability, open source, and long publication history 35 . The PC1D tool is used to simulate the www.nature.com/scientificreports/ optoelectrical properties of the ZnSe-CdSe Solar cell with inset parameters listed in Table 1 [36][37][38][39][40][41] . The standard solar radiation and light intensities are AM 1.5 and 0.1 W/cm 2 (one sun) at 25 °C temperature.

Result and discussions
Band alignment and band offset. The band alignment (BA) and band offsets (BO) 42 have a crucial role in the light reflectance, the transmission of photo charge carriers, and hence the efficiency of the solar cell 5,43 .
There are three types of band alignment: (a) Type I (Straddling gap): Conduction band (CB) and valence band (VB) of the second is lower and higher than that of the first resulting in its band gap being narrower than that of first. (b) Type II (Staggered gap): Both the CB and VB of the second are lower than the first. (c) Type III (Broken gap): There is an overlapping of the CB of the second to the VB of the first resulting in the zero band gap between them.
The schematic band-alignment diagram of the ZnO-ZnSe-CdSe solar cell is shown in Fig. 2. Both the alignment is of type II showing the migration of photoelectrons from both junctions onto the ZnO. Here, the electrons are excited to the CB upon the incidence of photon on the substrate thereby creating a hole in the VB. Photo-generated charge carriers were separated under the illumination in semiconductors. The electrons excited by incident light jumped to the conduction band (CB), while the holes were left in the valence band (VB). The excited electrons are transferred from more to less negative potential in CB and the hole created is from more positive to less positive potential in VB 44 . Together with the electrons emitted by ZnO, they flow onto the external circuit. On the other end, the holes of ZnO in its VB migrate to the VB of ZnSe and then together to the VB of CdSe. This system has enhanced PEC due to the enormous light harvesting capacity of CdSe and the novel band  As the two energy bands of the semiconductors are aligned, interaction occurs, and a continuous Fermi level is maintained throughout the combination due to the discontinuous band structure. This relative alignment is band offset. The interface and bulk properties give the band offset and can be modified according to them 46 . Further, in the heterovalent junctions, the band offset is affected by geometry, orientations, interface bonds, and charge transfer between them 47 . The band discontinuity (difference in the bandgap of the valence band and conduction band) and built-in potential (the bands bend at the interface due to an imbalance of charges in the two semiconductors) gives the band offset following Poisson's equation. Figure 2 shows the conduction band offset (CBO) and valence band offset (VBO) along with the electron affinity and band gap across the junction interface. The central or buffer layer has an energy gap of 2.82 eV and an electron affinity of 0.64 eV. It has positive BVO and negative CBO showing a reduction in carrier recombination. It has activation energy larger than that of the absorber 46 After the incidence of light on the surface of a material, a part of it is reflected which reduces the absorption and transmission of the photons whose energy depends on the bandgap of the material. If the energy of reflected photos coincides or is aligned with the material's conduction and valence band edges, the carrier concentration is increased. There is a transfer of electrons or holes thereby reducing the recombination process. So, the BO in the heterojunctions or interfaces between the different materials will reduce the transmission and increase the recombination process 42 . The BA or BO in the interface depends on the surface coating, interfacial layers, or doping which after optimization enhances transmission and reduces recombination for the higher efficiency of optoelectronic devices 49 .

Impact of the thickness of absorber and window layers. The thickness of the absorber and window
layer plays a crucial role in the solar cells' performance. As the thickness of the absorber layer increases, it traps more solar radiation thereby generating more charge carriers 50 . Whereas the window layer in combination with the absorber layer forms a p-n junction in a heterojunction thin-film solar cell to get a wider bandgap with smaller thickness and series resistance 51 . The thickness affects I sc , V oc , PCE & FF of the PV cell and is considered in the range of 0.5-3 µm for the absorber and from 10 to 100 nm for the window layer. The increases in absorber layer increase I sc from 0.791 to 1.638 A as in Fig. 3a. It is due to more photons being absorbed thereby producing more electron pairs at the higher thickness and hence producing more photoelectric current 52 . The V oc decreases from 0.813 to 0.800 V with an increase in the thickness of the absorber layer as in Fig. 3a due to more carrier recombination at higher thickness 19 . Similarly, the efficiency increases but the fill factor (FF) decreases with the thickness of the absorber layer due to more carrier recombination at higher thickness. The value of  Consequently, the value of I sc increases from 1.392 to 1.628 A and the value of V oc decreases from 0.807 to 0.778 V with an increase in the thickness of the window layer as in Fig. 4a. Also, the efficiency increase with increases in the window layer and FF decreases at higher thickness. The value of efficiency increases from 9.43 to 10.51% while FF decreases from 83.89 to 82.91% by varying the thickness of the window layer from 10 to 100 nm as in Fig. 4b. The optimized values of I sc = 1.404 A, V oc = 0.805 V, PCE = 9.473%, and FF = 83.79% have been observed at the optimized thickness of 50 nm of the window layer. The optimization of the thinner thickness of layers of materials of the solar cell helps to reduce the cost of fabrication.
Regarding the window layer of ZnSe, it minimizes the reflection loss by allowing the incident light toward the absorber layer. The layers are so optimized that they allow maximum photons through them. Similarly, the relatively thicker ZnSe layer, can absorb some percentage of the incident, and produce charge carriers that contribute to photocurrent generation which ultimately increases the current for better efficiency of the cell 53 . In order for achieving the thinner device structure as was done by Rickus 1982 with the material used, and the trade-off between transmission and absorption, the thickness of the window layer was taken to 50 nm. The relatively thicker single-layered ZnSe for higher photocurrent density leads toward the innovative application 6,54 . In Fig. 4b, the increasing efficiency with the thickness of the absorber without saturation may be due to consideration of range within the absorption limit. The saturation is reached only when the absorbing material does not contribute to fresh light absorption. Rather, it reabsorbs the already absorbed photons 55 . The recombination of the charge carriers, longer diffusion length, series resistance, and material quality may play a role in not reaching the system into saturation.
The sharp increase in current and efficiency (Fig. 4a,b) beyond 80 nm shows the unusual and unexpected behavior of the solar cell even after the repeated simulation. As per convention, we cannot consider the thickness of the window layer more relative to the active or absorber layer. Simultaneously, there was a sharp decrease in fill factor due to which we did not go beyond 80 nm. The sharp increase might be due to the limitation of the PC1D simulator, or the junction can have a breakdown 56 thereby breaking all the bonds thereby producing a large number of electron-hole pairs. However, this behavior is well stated by Sun et al. 2012 who have fabricated a ZnSe layer of nearly 40 nm thickness. They have found that the ZnSe single layers show eminently larger photocurrent density, remarkably higher incident photon-to-current efficiency (IPCE) of about 42.5% (bulk counterpart has 0.25%) with much better photo-stability due to the combined effect of morphology and electronics on a macroto-micro scale 54 . They have shown unique and unusual electronic structures for ultrathin thickness along with  www.nature.com/scientificreports/ their higher carrier mobility (t = d2/k2D (d is the particle size, k is a constant, D is the diffusion coefficient of electron-hole pairs) 57 and well-connected grain boundary 58,59 . The charge transfer resistance of the four atomic layers was lowest resulting in much higher carrier transport with a low corrosion rate 54 . Their synergistic surface distortion leads them to photostability. The contact with ZnO, ITO (Indium tin Oxide), etc. eases the electron to flow in the external circuit. This is not possible with the bulk counterpart of the ZnSe or in the presence of molecules. They have examined the distorted surface with X-ray absorption fine structure (XAFS) showing their unique and excellent structural stability, enhanced photoconversion efficiency, and photostability. 2.14 mAcm −2 of photocurrent density was achieved which is 195 times higher than that of its bulk form. They have reported their result mentioning that this behavior of the ZnSe thin layer has opened new avenues for bringing on a series of unprecedented excellent properties 54 .
In their experiment, the ZnSe was in contact with ITO. Stolarska et al. in 2017 found that ZnO is a robust alternative material for ITO replacement regarding environmental load and energy efficiency of the deposition process through the life cycle assessment technique. It is also crucial for sustainable transparent conductive oxide layer production. It is called a life cycle assessment (LCA) technique 60 . The result obtained in our simulation work almost agrees well with the above literature.

Impact of the doping concentration of absorber and window layers.
The performance of the solar cell depends on the doping concentration on the different layers of the solar cell 61 . In this work, the CdSe absorber layer is of p-type doping and the ZnSe window layer is of n-type doping. The impact of the doping concentration of the absorber and window layer on electrical properties like current & power has been analyzed by varying 1 × 10 16 -1 × 10 20 cm −3 . The highest value of current (I) = 1.402 A and power (P) = 0.952 W has been obtained at optimized doping concentration 1 × 10 17 cm −3 in the absorber layer as in Fig. 5a. Similarly, the highest value of current (I) = 1.404 A and power (P) = 0.901 W has been obtained at optimized doping concentration 1 × 10 17 cm −3 in the window layer as in Fig. 5b. The optimized value of doping concentration 1 × 10 17 cm −3 was well satisfied in both cases of the absorber and window layer.
The drastic reduction in performance caused by higher doping concentration in degenerate semiconductors is mainly due to the increased carrier scattering, Auger recombination, Fermi level pinning, non-ideal band structure, etc [62][63][64] . The optimization of the doping concentration is necessary for a balanced increasing carrier concentration and minimizing their harmful effect on the device performance. Further, optimization set compatibility between the input and output parameters for obtaining the overall performance of the system. So, greater care is taken at the time of optimization with respect to its objective, simulation modeling, performance metrics, and analyzing the results. The data are always reiterated, refined, and validated. The thickness of the absorber and window layer is focused for better performance by the whole system. The optimization of the window layer thickness depends on antireflection properties which are 50 nm (i.e. 50 × 10 -9 m) as the optimized thickness between 30 to 100 nm. The absorber of CdSe varied between 0.5 to 3 μm (i.e. 50 × 10 -6 m) and is optimized at 2 μm. Thickness is also related to the refractive index or optical energy density of the material. The window layer should ensure the absorption of the light and transmit it to the absorber layer. The chosen thickness facilitates the efficient extraction and collection of charge carriers from the absorber layer. At this thickness, the recombination rate is minimum and provides an effective path for the charge carriers to the electrode. This layer should have adequate electrical conductivity for efficient charge transport or low sheet resistance along with material compatibility. This thickness is feasible for the deposition of nearly 5 atomic layers of ZnSe over the CdSe substrate which we have practiced recently for the thinner MXene Oxide by Pulse Laser Deposition Method and analyzed the surface with RHEED Technique 65 .

Impact of the thickness of the ARC layer. The efficiency of solar cells can be enhanced by adding an
ARC layer which decreases the reflectance of solar radiation 66,67 . The impact of the thickness of the ARC layer on PV properties such as I sc , V oc , efficiency, and FF has been evaluated by varying the thickness from 31 to 107 nm and optimized at 78 nm. The numerical values of I sc = 1.76 A, V oc = 81 V, PCE = 11.92%, and FF = 83.5% have been achieved at the optimized thickness of the ARC as in Fig. 6.
The non-uniform thickness, defects, contaminations in the deposition chamber, traditional cleaning process, and poor film of ZnO affect the ARC by scattering the light 68 . For its improvement, the substrate should be compatible with respect to its thermal expansion coefficient, crystal structure, and the resulting strain and defects in the film. The nucleation and growth control are other measures for a good film 65 . A suitable deposition technique should be adopted under controlled temperature, pressure, and flow rate of the depositing material along with the rate of deposition. Consequently, the post-deposition annealing for quality crystal formation should be optimized. The thickness of the deposition should be uniform. The deposition technique that we recently used for the formation of an oxide layer in the preparation of MXene Oxide is preferable to meet this challenge 65 . The preparation of the single-layered ARC (SLARC) ZnO deposition on the ZnSe-CdSe solar cell can be achieved through a novel technique so far adopted in this field. The enhanced light trapping, wide bandgap engineering, simplified device structure, compatibility with ZnO as transparent conductive oxide, and scalability-versatility can lead to the novelty of this type of heterojunction solar cell.  www.nature.com/scientificreports/ The effect of increasing ARC thickness and electrical output depends on the device model and its material. The optimization of the ARC thickness can enhance the light absorption by optical interference, impedance matching, and reducing reflection loss 3 . However, care should be taken on the increased series resistance that increases the voltage loss, thereby reducing the fill factor and efficiency as a whole 3 . Consequently, the collection of charge carriers produced in the active layer will be reduced with the increase of the recombination rates or the longer carrier diffusion length. Similarly, there is the probability of blocking some portion of incident light from reaching the active layer 3 . With the advancement of technologies, the deposition of such a thinner layer of 50 nm (~ 5 atomic thickness) 69 is feasible for experiments as well which we have incorporated for a very sensitive MXene layer recently and analyzed the surface 65 .
Analysis of optoelectrical properties. The reflectance of solar radiation on the surface of solar cells plays a vital role to enhance the rate of generation of photocurrent. The ARC layer on the surface of the solar cell helps to absorb incident photons, reduces reflectance, and increases the I sc due to destructive interference 67,70 . The appropriate thickness of the ARC layer can only produce destructive interference. The thickness (d 1 ) of the ARC layer to get a quarter-wavelength coating of a transparent material is given by 71 , and, where η 1, η 0, andη 2 are the refractive indices of the coating material, air, and window layer respectively, and λ 0 is the wavelength of the incident light at a wavelength. By using Eqs. (1) and (2), we have calculated the thickness of the ZnO ARC layer. The reflectance spectra of different thicknesses of the ZnO ARC layer have been explored between 300 and 1000 nm wavelength. The average reflectance of 18.91%, 12.2%, 7.53%, 6.45%, 6.61%, and 8.07% at a thickness of 31 nm, 47 nm, 63 nm, 78 nm, 93 nm, and 107 nm respectively in the range of 400-1000 nm wavelength as in Fig. 7a. The minimum average minimum reflectance (R av ) of 6.45% has been obtained at 78 nm thickness of the ZnO ARC layer due to perfect destructive interference 72 . The I-V and P-V characteristics of solar cells with and without ARC layers have been analyzed with an optimized thickness of the ARC layer respectively as in Fig. 7b.  Table 2 and the optimized and recommended are in Table 3. Furthermore, the more detailed behavior of the solar cell has been analyzed by the external quantum efficiency (EQE) method in a specified range of wavelength (300-1000 nm) 73,74 . The EQE of solar cells has been observed at about 90% at a particular wavelength at 470 nm (visible light range) as in Fig. 8b. Therefore, this simulation study of optoelectrical properties manifests that the ZnSe-CdSe Solar cell-based with ZnO ARC layer is costeffective attaining an efficiency of nearly 12% and stimulated with visible light.

Conclusion
The numerical analysis of optoelectrical properties of ZnSe-CdSe solar cells has been successfully investigated by using the PC1D simulation tool. We have studied the band alignment and band offset across the heterojunction of the solar cell. The photovoltaic properties have been optimized by varying thicknesses of the p-CdSe absorber layer, n-ZnSe window layer, and ZnO ARC layer and also investigated the doping concentration effect on solar cell performance. The minimum average minimum reflectance (R av ) of 6.45% has been obtained at 63 nm thickness of the ZnO ARC layer due to perfect destructive interference. The ZnO/ZnSe/CdSe solar cell exhibited a high efficiency of 11.98% with I sc = 1.72 A, V oc = 0.81 V, and FF = 90.8% at an optimized thickness of 2 μm absorber layer, 50 nm window layer, and 78 nm ARC layer. The efficiency and short circuit current increase rapidly and unusually after 80 nm thickness of the ZnSe window layer indicating the possibility of the production of a large number of electron-hole pairs due to the combined effect of morphology and electronics on a macro-to-micro scale. which is in good agreement with the previous literature. Thus, the optoelectrical properties from this study exhibited that the ZnSe-CdSe Solar cell-based with ZnO ARC layer is cheap, visible light stimulated, and efficient to fabricate high-performance solar cells within the optimized limit.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.